Direct thermal extraction uses a combination of heat and a flowing stream of inert gas to extract and then trap, volatile and semi-volatile analytes from solid materials, prior to analysis by GC or GC-MS. It can be a very attractive alternative to solvent extraction.
Compared to Soxhlet extraction for example:
Mass transfer in solid materials is much slower than in either the liquid or gas phase and the degree to which liquid extraction can be speeded-up by the use of heat is limited by the need to keep the (often volatile) solvent in the liquid phase. In the case of thermal extraction, however, the only limitation is the thermal stability of the analytes and target compounds. The significance of this is that if a material is heated to a temperature above its glass transition point, then its properties become more like those of a liquid and the mass transfer rates increase steeply. This enables analytes to be extracted from the matrix much more readily. Polymers are one class of compounds, where the glass transition temperatures can be easily achieved during the process of thermal extraction.
Thermal Desorbers are designed to release analytes from the surface of adsorbents by the application of a hot gas stream, so it is clear that they match the requirements of thermal extraction to some degree. It is true that if the target analytes are all volatile organic compounds then the technique is likely to work with few problems, however, since thermal extraction is used as an alternative to solvent extraction, in most cases, the suite of analytes will include some compounds that are rather involatile. Successful analysis of these high boiling compounds will depend upon the flow path for the extraction gas being both sufficiently hot and as short as possible to minimise losses.
Since the release of analytes into the extraction gas is relatively slow, then the volume of the extraction gas is larger that the GC column can accommodate directly. As a consequence, it is necessary to trap the analytes and release them into a small volume of carrier gas. There are two approaches available. Conventional thermal desorption would use a secondary adsorbent bed, taking the analytes through a second cycle of adsorption and thermal desorption. The only difficulty with this approach is the need to use an adsorbent that is strong enough to quantitatively trap the most volatile analytes but weak enough the readily desorb the least volatile compounds. A better approach is to employ cryogenics trapping. A trapping temperature of -150oC is sufficient to trap the most volatile compounds - without the need to resort to an adsorbent. This in turn means that the least volatile analytes can be released cleanly from the Cryotrap and ensures quantitative transfer to the column.
The detection limits needed for this type of analysis are often in the low ppb region to achieve this, with a reasonable margin for error requires that all of the target analytes be transferred to the column - without splitting. Cryo trapping permits this. During the extraction phase, a relatively high flow can be employed since the bulk of the gas can be vented via a split line located after the cryo trap and after the analytes have been quantitatively removed from the extraction gas. The split line can then be closed prior to transfer of the analytes to the column.
The GERSTEL TDS Thermal Desorber is widely used for this application as it has characteristics that are uniquly suited to Thermal Extraction. It has the capacity to extract relatively large masses of matrix material, is able to perform splitless transfer of analytes the column and permits the analysis of a wide analytes covering a wide volatility - from C2 - C40.
An application note discussing the thermal extraction of polymers can be found in on our resource site: Anatune.org - search for "Thermal Extraction".
If you want to know more please call us on 01954 212909, or email: enquiries@anatune.co.uk, and one of our technical staff will be happy to advise you.
Compared to Soxhlet extraction for example:
- The extraction process is faster (solvent extraction can take many hours)
- Limits of detection can be excellent (down to low ppb)
- Matrix interference can be reduced compared with solvent extraction
- The risk of analytes being masked by the solvent is eliminated
- There are no problems with target analytes (such as anti-oxidants) also being present in the solvent
- A reduction in the use of hazardous (and expensive) high purity, solvents
Mass transfer in solid materials is much slower than in either the liquid or gas phase and the degree to which liquid extraction can be speeded-up by the use of heat is limited by the need to keep the (often volatile) solvent in the liquid phase. In the case of thermal extraction, however, the only limitation is the thermal stability of the analytes and target compounds. The significance of this is that if a material is heated to a temperature above its glass transition point, then its properties become more like those of a liquid and the mass transfer rates increase steeply. This enables analytes to be extracted from the matrix much more readily. Polymers are one class of compounds, where the glass transition temperatures can be easily achieved during the process of thermal extraction.
Thermal Desorbers are designed to release analytes from the surface of adsorbents by the application of a hot gas stream, so it is clear that they match the requirements of thermal extraction to some degree. It is true that if the target analytes are all volatile organic compounds then the technique is likely to work with few problems, however, since thermal extraction is used as an alternative to solvent extraction, in most cases, the suite of analytes will include some compounds that are rather involatile. Successful analysis of these high boiling compounds will depend upon the flow path for the extraction gas being both sufficiently hot and as short as possible to minimise losses.
Since the release of analytes into the extraction gas is relatively slow, then the volume of the extraction gas is larger that the GC column can accommodate directly. As a consequence, it is necessary to trap the analytes and release them into a small volume of carrier gas. There are two approaches available. Conventional thermal desorption would use a secondary adsorbent bed, taking the analytes through a second cycle of adsorption and thermal desorption. The only difficulty with this approach is the need to use an adsorbent that is strong enough to quantitatively trap the most volatile analytes but weak enough the readily desorb the least volatile compounds. A better approach is to employ cryogenics trapping. A trapping temperature of -150oC is sufficient to trap the most volatile compounds - without the need to resort to an adsorbent. This in turn means that the least volatile analytes can be released cleanly from the Cryotrap and ensures quantitative transfer to the column.
The detection limits needed for this type of analysis are often in the low ppb region to achieve this, with a reasonable margin for error requires that all of the target analytes be transferred to the column - without splitting. Cryo trapping permits this. During the extraction phase, a relatively high flow can be employed since the bulk of the gas can be vented via a split line located after the cryo trap and after the analytes have been quantitatively removed from the extraction gas. The split line can then be closed prior to transfer of the analytes to the column.
The GERSTEL TDS Thermal Desorber is widely used for this application as it has characteristics that are uniquly suited to Thermal Extraction. It has the capacity to extract relatively large masses of matrix material, is able to perform splitless transfer of analytes the column and permits the analysis of a wide analytes covering a wide volatility - from C2 - C40.
An application note discussing the thermal extraction of polymers can be found in on our resource site: Anatune.org - search for "Thermal Extraction".
If you want to know more please call us on 01954 212909, or email: enquiries@anatune.co.uk, and one of our technical staff will be happy to advise you.